Hydrogen sulfide gas has a number of undesirable properties. At low concentrations (i.e., less than 10 parts per million), the gas has a strong odor of "rotten eggs". Paradoxically, at concentrations greater than 100 parts per million (ppm), the gas can no longer be smelled, but it will induce dizziness. Concentrations of greater than about 500 ppm can be fatal to humans. Hydrogen sulfide can also be oxidized to sulfur dioxide, a chemical that contributes to acid rain.
Hydrogen sulfide (H.sub.2 S) gas can be released by a variety of sources. Biologically-generated hydrogen sulfide is generally the product of anaerobic digestion of organic matter, and can be released by sewage treatment facilities, solid waste landfill, paper mill waste, cattle feed lots, poultry farms, and other industries employing anaerobic digestion for processing. H.sub.2 S is also released by oil drilling operations, oil refinery operations, and operations utilizing geothermal brines, such as power generation.
Geothermal brines are of interest for several reasons. Geothermal formations containing brines heated to high temperature can be accessed by conventional drilling technology and are found in a variety of locales, including California. When a geothermal brine is brought to the surface and allowed to "flash", live steam is generated that can be used to drive turbines for the production of electricity. The brines also contain large quantities of commercially valuable metals, for example, lead, silver and zinc. However, when geothermal brines are flashed, they also release non-condensable gases, including H.sub.2 S. H.sub.2 S concentrations in non-condensable gas streams from geothermal brines can be quite high, reaching levels in excess of 4000 ppm.
Abatement (i.e., removal or transformation of H.sub.2 S) can be accomplished by chemical or biological means. The object of H.sub.2 S abatement systems is to convert H.sub.2 S gas into elemental sulfur, a solid that can easily be collected. An older chemical process for H.sub.2 S abatement, the Claus process, uses heat and oxygen to oxidize H.sub.2 S to elemental sulfur. This process is not particularly efficient, however, and produces sulfur dioxide. Thus, a catalytic step is required to react the remaining H.sub.2 S and the sulfur dioxide to form elemental sulfur. Dow Chemical Corporation and U.S. Filter Engineered Systems are vendors for newer technologies (sold under the tradenames SulFerox.RTM. and ARI LO-CAT II.TM., respectively). Both of these technologies utilize proprietary iron catalysts in liquid reactors for the conversion of hydrogen sulfide to elemental sulfur.
Biological technologies for H.sub.2 S abatement are also available. Sulfur bacteria (i.e., bacteria that are able to metabolize sulfur compounds) can be utilized in "biofilter" or "bioscrubber" reactors to oxidize H.sub.2 S to sulfates. Biofilter and bioscrubber plants are normally designed to produce elemental sulfur. In this process, H.sub.2 S gas is passed over the bioreactor bed under aerobic conditions, leading to reduction of the H.sub.2 S to elemental sulfur that is deposited intracellularly and extracellularly as a solid in clumps or granules. This elemental sulfur must be mechanically removed periodically from the bioreactor bed or the reactor will become clogged. The deposited sulfur, once removed, may be disposed of or utilized in industrial and/or agricultural processes. The further oxidation of sulfur by biofilter or bioscrubber reactors has been considered undesirable. Sulfuric acid, a strong acid, is the principle product of further oxidation of elemental sulfur by sulfur bacteria. Sulfuric acid has not been a desired product because it is highly corrosive (the sulfuric acid produced by sulfur bacteria generally has a pH of 1 to 2). Moreover, sulfate ions are so undesirable that bioreactor technology is available (for example, from THIOPAQ Sulfur Systems BV) for the biological conversion of sulfate to elemental sulfur. In this technology, anaerobic bacteria are employed to reduce sulfate to H.sub.2 S, which is then oxidized by aerobic sulfur bacteria to form elemental sulfur.
Lanting et al., "Biological Removal of Hydrogen Sulfide from Biogas", presented at the 46th Annual Purdue Industrial Waste Conference, May 14-16, 1991, disclose methods for the use of biological systems for the abatement of H.sub.2 S from biological sources. The disclosed methods relate to the biological abatement of H.sub.2 S by conversion into elemental sulfur. In this method, H.sub.2 S-containing gas is passed over a support medium coated with sulfur bacteria. Water and a nutrient fluid are circulated through the bioreactor bed. H.sub.2 S is oxidized to elemental sulfur and sulfate, although elemental sulfur is the desired end product. The production of sulfates is noted, and actions to reduce or eliminate sulfate production are suggested. In the Lanting method, elemental sulfur is removed from the bioreactor by periodically flushing the reactor.